This opens a wormhole and sends appropriate JSON down
it to a tahoe-gui using a wormhole server running on
tahoe-lafs.org
The other end uses the 'tahoe create-node' command (with
new --join option) to read the configuration JSON from
a 'tahoe invite' command
This uses Read-The-Docs (sphinx/docutils) references exclusively, but adds a
README.md for GitHub viewers to remind them that the links there won't
work (closes ticket:2835).
It also fixes all the dangling references and other Sphinx warnings.
The "Preparation" section of docs/magic-folder-howto.rst was removed, since
this feature has since been merged to trunk.
